Jet perforating guns are used downhole in a cased borehole for perforating the casing at a location adjacent to a hydrocarbon containing formation. Perforating guns include a charge carrier with shaped charges which shoot through the cased borehole and into the formation. The shaped charges are generally loaded into the charge carrier through one end of the carrier or through ports passing through the side of the carrier. Generally the shaped charges are loaded one charge at a time, and port wrenches are used to install the charges in the ports. This requires substantial labor for the loading of the shaped charges. The ports are also used to hold the charges in position and to fix their location within the charge carrier of the perforating gun. Upon detonation of the perforating gun, the shaped charges then shoot through the ports.
Perforating guns must be hermetically sealed to prevent fluids from leaking into the charge carrier and damaging the charges so as to prevent them from detonating. Thus, many seals are required in prior art guns. Rubber jackets are placed around the charges so that the charges will be sealed within the ports. However, these rubber jackets may melt in a high temperature environment such that the seal around the ports is lost if the rubber jackets melt or deform. If such were to occur, the alignment of the charges in the carrier would also be lost. Further, it is possible in a high pressure well that the integrity of the seals of the ports may be broken.
Although charges are often located one above the other in a staggered fashion in the carrier, it is often desirable to load the charges in a cluster circumferentially spaced within the charge carrier. The prior art methods of loading such a plurality of charges in a cluster is very cumbersome. Further, it is necessary that the shaped charge be within a reasonable distance of the prima cord or the charges will not detonate. Thus, it is necessary to squeeze the shaped charge down onto the prima cord in the center of the charge carrier to insure detonation.
In some very small perforating guns with a maximum diameter of two inches, the ports have been eliminated so that the charges shoot directly through the sidewall of the charge carrier. The charges for such small perforating guns are mounted on a member in aligned relationship outside the perforating gun housing and then lowered as a unit into the interior of the gun housing. It is necessary that all the charges be very rigid and locked into place so that they can be lowered into the charge carrier housing as a unit. Thus, small guns require a solid alignment system.
In a high shot density gun, often an aluminum tube is used. Holes are drilled into the sides of the tube for receiving and housing individual shaped charges. Once the charges are inserted into the holes, the charges are banded onto the tube. Further, it is necessary that a hole be drilled all the way through the tube from one end to the other in order to house the prima cord. Such milling, drilling, and machining of the aluminum tube is very cumbersome and expensive.
It is often desirable to obtain a specific perforating pattern by orienting each of the shaped charges to discharge in a specific direction relative to the other charges. The shaped charges are usually placed as close to one another as possible, and each charge therefore is successively rotated about the longitudinal axial centerline of the carrier respective to the next adjacent charge. Vertically spaced adjacent charges of a cluster may be oriented to discharge along radials which diverge 60.degree. apart, for example; and the individual charges of a cluster may be arranged to discharge 120.degree. apart, for example.
As pointed out in U.S. Pat. No. 4,194,577, it is sometimes desirable to orient shaped charges to fire in a downward direction; and, as pointed out in U.S. Pat. No. 4,140,188, it is sometimes desirable to orient the charges to fire radially away from the borehole in a specific pattern. Therefore, it is often desirable to arrange shaped charges in a manner so that when the charges are detonated, they do not interfere with one another, and yet they are grouped as close as possible so as to attain a very dense perforation pattern. In this instance, it is desirable to be able to conveniently and economically mount the shaped charges within a gun housing in a foolproof and structurally safe manner, whereby each of the charges is properly aligned respective to the gun axis and to the borehole axis so that when the gun is detonated, the resultant perforating job achieves the desired result.
A perforating gun which attains the above desirable goals is the subject of the present invention.